Vacuum Distillation Assembly, Reduced Pressure

Quick Answer: A vacuum distillation apparatus is a glass assembly connected to a vacuum pump so that liquids distil at reduced pressure and therefore at a lower temperature. It is used for high-boiling liquids and for compounds that would decompose if heated to their normal boiling points.

Lower Pressure, Lower Boiling Point

A liquid boils when its vapour pressure equals the pressure above it. Reducing the pressure in the apparatus therefore lowers the temperature needed, and under a good vacuum the boiling point of a high-boiling organic liquid can fall by well over a hundred degrees. That lets compounds such as high-boiling esters, amines or oils be distilled without charring, and lets heat-sensitive products be purified at modest bath temperatures.

A vacuum set differs from an ordinary one in several ways. A Claisen adapter gives a second neck, used for a fine capillary air bleed or to keep splashes out of the condenser. Boiling chips do not work under vacuum, so smooth boiling comes from the capillary bleed or from magnetic stirring. The receiving end has a vacuum adapter with a hose connection, often with a multi-limb receiver, sometimes called a cow or pig, that turns to collect separate fractions without letting air in. A cold trap and a manometer sit between the apparatus and the pump.

Only round-bottomed, undamaged flasks may be evacuated; flat-bottomed and conical flasks can implode. The listing does not state which parts, joint size or flask capacity are included. For work at atmospheric pressure see the distillation apparatus, and for distilling off extraction solvent the recovery assembly.

Care & Handling

Inspect every piece for star cracks and deep scratches before evacuating, and set aside any doubtful glass.
Evacuate before heating; when finishing, remove the heat and let the flask cool before admitting air slowly.
Use a safety screen around the apparatus and wear eye protection throughout.
Grease every joint thinly and fit joint clips so the assembly holds its vacuum.

Specifications

Principle	Distillation at reduced pressure, lowering the boiling temperature
Characteristic parts	Claisen adapter, capillary bleed or stirring, vacuum adapter, fraction receiver
Also needed	Vacuum pump, cold trap, manometer, safety screen
Flasks	Round-bottomed only under vacuum
Parts list, joints and capacity	Confirm at enquiry

Applications

Purifying high-boiling liquids such as aniline or nitrobenzene in organic chemistry courses
Distilling heat-sensitive intermediates and natural products in research laboratories
Separating fractions of oils and plasticisers that would decompose at their atmospheric boiling points
Recovering high-boiling solvents such as dimethyl sulfoxide or dimethylformamide

Why Choose LabEquip

University organic laboratories, pharmaceutical development teams and research groups use a vacuum distillation apparatus for compounds that cannot survive atmospheric distillation. LabEquip supplies assemblies within its chemistry lab equipment; a parts list with the joint size can be sent from the contact page.

Frequently Asked Questions

Why don’t boiling chips work under vacuum?

Boiling chips work by releasing trapped air as starting points for bubbles. Under vacuum that air is drawn out almost at once, so the chips stop working. A fine capillary air bleed or magnetic stirring provides smooth boiling instead.

Why must round-bottomed flasks be used?

A sphere spreads the pressure of the atmosphere evenly over its surface. Flat bases and conical shapes have weak points where stress concentrates, and they can implode violently when evacuated.

How do I estimate the boiling point at reduced pressure?

Use a pressure-temperature nomograph or published vapour pressure data for the compound. The manometer reading in the line tells you which pressure to look up, which is one reason a manometer should always be fitted.

What is the cold trap for?

It condenses solvent vapour before it reaches the pump. This protects the pump oil or diaphragm from contamination and keeps the vacuum steady.

What is a cow receiver?

It is a receiver adapter with several limbs, each carrying a small flask. Rotating it on its joint directs the distillate into a fresh flask for each fraction without breaking the vacuum.

In what order should the apparatus be shut down?

Remove the heat and let the flask cool, then slowly let air back in through the bleed or a stopcock, and only then switch off the pump. Stopping the pump first can suck pump oil or water back into the apparatus.

Last Updated: September 2026